- Pregnant Rhesus monkeys treated with 131I at midgestation become hypothyroid and produce fetuses without demonstrable thyroid tissue. In an effort to prevent both maternal and fetal hypothyroidism, we treated 131I-treated pregnant monkeys with 3,5-dimethyl-3'-isopropyl-L-thyronine (DIMIT), a thyroid hormone analog with structural changes which facilitate placental transfer. Five pregnant monkeys were treated with 131I (mCi/kg) at 83-87 days of gestation. One week later, three monkeys were started on treatment with DIMIT (10 micrograms kg-1 day-1, im) and two on im L-T4 (2 micrograms kg-1 day-1). Treatment was continued until delivery by Caesarian section at 152-157 days of gestation. None of the DIMIT-treated mothers became clinically hypothyroid, nor did they have elevated serum TSH concentrations despite low serum levels of T3 and T4. T4-treated mothers were also maintained clinically and biochemically euthyroid. At delivery, infants of DIMIT-treated mothers had normal respiratory function and skeletal maturation. Basal and TRH-stimulated TSH concentrations were suppressed in two of the three infants. By contrast, both T4-treated infants resembled untreated cretinous newborns and died soon after delivery from respiratory failure. Serum TSH concentrations were elevated and skeletal maturation was markedly delayed in these animals. We conclude that DIMIT administration to 131I-treated monkeys prevents clinical and biochemical hypothyroidism in the mother and prevents the major clinical manifestations of cretinism in the fetus.